1. Field of the Invention
The subject invention relates generally to electrical circuit fabrication and, more particularly, to a method and apparatus for determining the integrity of wirebonds formed by wirebonding apparatus.
2. Description of Related Art
Presently, wirebonding to integrated circuitry is performed by several methods, including ball bonding and wedge bonding. A typical ball bonding operation forms a "ball bond" to an IC and a "stitch bond" to a second bond site, followed by a "stitch tear."
Various defects are known to occur in such ball bonding procedures, including lifted balls, misregistered balls, improper ball size, cratering, and defective stitches. The "lifted" ball bond does not adequately weld to the die bond pad or conductor. Contamination or a misapplication of the bond force component is usually the cause of lifted balls. The misregistered ball is placed disproportionately on the ball bond pad site and an adjacent material, typically either silicon dioxide glassivation or dielectric. Hence, only a portion of the entire ball bond area is actually welded during the bond cycle. Additionally, the ball bond size may be either too small or too large.
A cratered die bond pad occurs explosively during the ball bond cycle. Adequate coupling takes place in the early stages of the ball bond cycle, then suddenly, high shear forces cause the underlying silicon to break away. The ball bond is thus left unsupported.
Defective stitches include cut stitches and lifted stitches. With a cut stitch, the wire tail has already been released from the bond site due to overbonding. On the other hand, with the lifted stitch, the wire at the stitch has been underbonded and a greater force is required to separate the wire tail from the stitch end of the bond.
Given the foregoing and other potential defects, wirebonds formed by wirebonding apparatus are typically required to go through laborious and expensive testing. For example, current military requirements specify internal visual inspection for determining the integrity of wirebonds and other microcircuit interconnections. Visual inspection of wirebonds has become a costly task, for example, requiring approximately eight inspection stations operating two shifts per day at some facilities. Production operators at these stations may review hundreds of thousands of initial build wirebonds and rework build wirebonds in a normal week. Some military requirements also require a 100% nondestructive wire pull test of all wirebonds. Wire pull takes longer than visual inspection and is prone to cause damage to the product. Another useful test is the simple determination of the presence of a wire in the bond tool during bonding, e.g., during wirelooping and stitch bond formation.
All of the foregoing extensive testing results in detection of a relatively small number of defective wirebonds. For example, the quantity of defective wirebonds on initial-build hybrids at one typical facility was calculated at 0.24% of the total quantity of wirebonds installed. In this instance, the capability to identify bonds likely to be visually marginal would effectively reduce the quantity of wirebonds required to be inspected and dispositioned for rework from 16,250,000 bonds to only 39,000 bonds on initial build hybrids